Equation of state of MgGeO3 perovskite to 65 GPa: comparison with the post-perovskite phase

Abstract

The equation of state of MgGeO3 perov- skite was determined between 25 and 66 GPa using synchrotron X-ray diffraction with the laser-heated diamond anvil cell. The data were fit to a third-order Birch-Murnaghan equation of state and yielded a zero- pressure volume (V0) of 182.2 ± 0.3 A ˚ 3 and bulk modulus (K0) of 229 ± 3 GPa, with the pressure derivative (K¢0 =( ¶K0/¶P)T) fixed at 3.7. Differential stresses were evaluated using lattice strain theory and found to be typically less than about 1.5 GPa. Theo- retical calculations were also carried out using density functional theory from 0 to 205 GPa. The equation of state parameters from theory (V0 = 180.2 A ˚ 3 , K0 = 221.3 GPa, and K¢0 = 3.90) are in agreement with experiment, although theoretically calculated volumes are systematically lower than experiment. The prop- erties of the perovskite phase were compared to MgGeO3 post-perovskite phase near the observed phase transition pressure (~65 GPa). Across the tran- sition, the density increased by 2.0(0.7)%. This is in excellent agreement with the theoretically determined density change of 1.9%; however both values are larger than those for the (Mg,Fe)SiO3 phase transition. The bulk sound velocity change across the transition is small and is likely to be negative (-0.5(1.6)% from experiment and -1.2% from theory). These results are similar to previous findings for the (Mg,Fe)SiO3 sys- tem. A linearized Birch-Murnaghan equation of state fit to each axis yielded zero-pressure compressibilities of 0.0022, 0.0009, and 0.0016 GPa -1 for the a, b, and c axis, respectively. Magnesium germanate appears to be a good analog system for studying the properties of the perovskite and post-perovskite phases in silicates.